Metal oxide varistors (MOV) are state of the art protection devices that are directly connected across the terminals of a power conditioning circuit. For power conditioning circuits such as GTO and SCR converters they provide for component protection from transients and temporary overvoltages. In many applications large capacitors are also directly connected across the terminals and parallel with the MOV to provide for fast transient overvoltage protection.
The MOV device or arrestor operates in a standby mode under maximum steady state voltage conditions and in a voltage clamping mode under transient conditions. The MOV device has a maximum-continuous operating voltage (MCOV) characteristic. This greatly reduces the steady state peak voltage relative to the clamping level for the direct connected MOV device. In high power GTO and SCR converters large harmonics caused by switching are present. These cause increased heating and standby power dissipation requirements for the MOV which further reduce the steady state peak voltage relative to the clamping level.
The MCOV characteristic can be increased by cooling. Cooling, however, increases the standby power dissipation and tends to force the current into a narrow channel. The MCOV characteristic can also be increased by adding more columns of MOVs. However, a large number of parallel columns of MOVs is impractical.
A figure of merit for the MOV clamping circuit is the voltage clamping ratio (VCR). This is expressed as a ratio of voltage clamped (at a given coordinating current) to the peak maximum continuous operating voltage. For a given peak MCOV the lowest protection level is defined by the voltage clamping ratio. Where V.sub.m (ac) rms is the maximum continuous operating voltage, the recurrent peak voltage is .sqroot.2 times this value. For non-sinusoidal waveforms the peak voltage of the fundamental and harmonics must be below the recurrent peak voltage rating. A voltage clamping ratio approaching 1.0 is ideal. However, direct connected MOVs provide a clamping circuit that has a minimum voltage clamping ratio of about 1.6 for typical coordinating currents.
In circuit design, the voltage withstand capability of the equipment is protected by a margin from transient and temporary overvoltage. Where added components are used to increase the voltage withstand capability, equipment cost increases. By decreasing the voltage clamping ratio of the protection circuit a cost saving may be achieved in the design of protected equipment.